Phobos
Phobos or Mars I is a small moon orbiting Mars about 9,337 km above the surface. Discovery Phobos was discovered by Asaph Hall Sr. on August 18, 1877. Hall also discovered Mars' other moon, Deimos. Physical Charecteristics Phobos is one of the least-reflective bodies in the solar system. Spectroscopically it appears to be similar to the D-type asteroids, and is apparently of composition similar to carbonaceous chondrite material. Phobos' density is too low to be solid rock, however, and it is known to have significant porosity. Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. Phobos is highly nonspherical, with dimensions of 27 × 22 × 18 km. Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos' gravity at its sub- and anti-Mars poles. Phobos is heavily cratered. The most prominent surface feature is Stickney crater, like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. The unique Kaidun meteorite is thought to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon. Orbital Charecteristics Phobos' unusually close orbit around its parent planet produces some unusual effects. As seen from Phobos, Mars would appear 6400 times larger and 2500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere. Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). Since it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°. As seen from Mars' equator, Phobos would be one-third the angular diameter of the full Moon as seen from Earth. Observers at higher Martian latitudes would see a smaller angular diameter because they would be significantly farther away from Phobos. Phobos' apparent size would actually vary by up to 45% as it passed overhead, due to its proximity to Mars' surface: for an equatorial observer, for example, Phobos would be about 0.14° upon rising and swell to 0.20° by the time it reaches the zenith. By comparison, the Sun would have an apparent size of about 0.35° in the Martian sky. Phobos' phases, in as much as they could be observed from Mars, take 0.3191 days (Phobos' synodic period) to run their course, a mere 13 seconds longer than Phobos' sidereal period. Solar Transits An observer situated on the Martian surface in a position to observe Phobos would see regular transits of the moon across the Sun. Phobos is not large enough to cover the Sun's disk, and so cannot cause a total eclipse. Fate Due to Phobos's low rotational rate relative to its orbital rate, tidal locking is decreasing its orbital radius at the rate of about 20 meters per century. In 11 million years it will either impact the surface of Mars or more likely break up into a planetary ring. Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically a Mohr-Coulomb body), it has been calculated that Phobos is currently stable with respect to tidal forces. But it is estimated that Phobos will pass the Roche Limit for a rubble pile when its orbital radius drops by a little over 2000 km to about 7100 km. At this distance Phobos will likely begin to break up and form a short lived ring system around Mars. The rings themselves will then continue to spiral slowly into Mars. Category:Moon